The applicant's conveyor belt packaging machines are known from practice in which a sealing device comprises a sealing bar and a diaphragm. To seal a bag, the diaphragm is subjected to pressure to press the sealing bar against a counter-pressure bar in order to heat-seal the bag neck clamped between the sealing bar and the counter-pressure bar. During the evacuation of the bag, the sealing bar is moved to an evacuation position in which the gap between the sealing bar and a counter-pressure bar is at a maximum. This is accomplished by evacuating the diaphragm, and by a device pressing the sealing bar against the diaphragm with springs. The diaphragm is connected to a vacuum pump, which is also connected to the vacuum chamber for evacuating the bag, via a supply line that is partially provided also outside of the vacuum chamber. If the supply line is leaking in a section between the vacuum chamber and the vacuum pump, then this leakage can cause the sealing bar not to reach its evacuation position and the gap between the sealing bar and the counter-pressure bar, and thereby the opening of the bag neck is reduced such that evacuation or minimizing the oxygen content is not sufficient to meet the specified minimum shelf life, especially for foods to be packaged, such as meat, so that returns and complaints can occur. Such leakage of the diaphragm or in the supply line can also lead to the intended sealing pressure not being reached, and thereby the quality of the sealing seam being reduced, without this being easily apparent to the user during a visual check.